The present invention relates in general to fabrication of semiconductor devices and in particular to an electron beam exposure system for writing a semiconductor pattern on a semiconductor substrate by an electron beam.
In the submicron patterning of semiconductor devices, the electron beam exposure system is suitable. The electron beam exposure system uses a finely focused electron beam for writing a semiconductor pattern on a semiconductor substrate and can achieve the resolution of less than 1 .mu.m without difficulty. On the other hand, the conventional electron beam exposure system has suffered from the problem of relatively low throughput because of the basic constraint of the system in that the semiconductor pattern is written in one stroke of the focused electron beam.
In order to improve the problem of low throughput, a technique of so-called block exposure is proposed. According to this procedure, the electron beam is shaped into a desired one of fundamental patterns of several, selectable semiconductor devices and the desired semiconductor pattern is written on the substrate as a consecutive repetition of "shots" of selected fundamental patterns. This block exposure technique is particularly suited for the fabrication of semiconductor devices such as memories in which a repetition of fundamental patterns is included.
FIG. 1 shows the construction of a conventional electron beam exposure system that uses the technique of block exposure. Referring to the drawing, the electron beam exposure system generally comprises an electron optical system 100 for producing and focusing an electron beam and a control system 200 for controlling the optical system 100.
The electron optical system 100 includes an electron gun 104 as a source of the electron beam. The electron gun 104 includes a cathode electrode 101, a grid electrode 102 and an anode electrode 103, and produces the electron beam generally in the direction of a predetermined optical axis O in the form of a spreading beam.
The electron beam thus produced by the electron gun 104 is passed through a shaping aperture 105a formed in an aperture plate 105. The aperture plate 105 is provided such that the aperture 105a is in alignment with the optical axis O and shapes the incident electron beam to have a rectangular cross section.
The electron beam thus shaped is received by an electron lens 107a that has a focal point coincident with the aperture 105a. Thereby, the incident electron beam is converted to a parallel beam and enters into an electron lens 107b that focuses the electron beam on a block mask 110. It should be noted that the lens 107b projects the image of the rectangular aperture 105a on the block mask 110. As shown in FIG. 2, the block mask 110 carries a number of fundamental patterns 1a, 1b, 1c, . . . of the semiconductor device pattern to be written on the substrate in the form of apertures, and shapes the electron beam according to the shape of the aperture through which the electron beam has passed.
In order to deflect the electron beam passed through the electron lens 107b and address the desired aperture, deflectors 111, 112, 113 and 114 are provided, wherein the deflector 111 deflects the electron beam away from the optical axis O in response to a control signal SM1. The deflector 112 in turn deflects back the electron beam generally in parallel to the optical axis O in response to a control signal SM2. After passing through the block mask 110, the deflector 113 deflects the electron beam toward the optical axis O in response to a control signal SM3, and the deflector 114 deflects the electron beam such that the electron beam travels coincident to the optical axis O in response to a control signal SM4. Further, the block mask 110 itself is movable in the direction perpendicular to the optical axis O for enabling the addressing of the apertures on the entire surface of the block mask 110 by the electron beam.
The electron beam thus passed through the block mask 110 is then focused at a point f1 that is located on the optical axis O after passing through electron lenses 108 and 116. There, the image of the addressed aperture on the block mask 110 is demagnified at the point f1. The electron beam thus focused is then passed through a blanking aperture 117a formed in a blanking plate 117 and further focused on the surface of a substrate 123 that is held on a movable stage 126, after passing through electron lenses 119 and 120 that form another demagnifying optical system. There, the electron lens 120 serves for an objective lens and includes various coils such as correction coils 120 and 121 for focusing compensation and astigmatic compensation as well as deflection coils 124 and 125 for moving the focused electron beam over the surface of the substrate 123.
It should be noted that the foregoing blanking aperture 117a is provided coincident to the optical axis O for establishing an alignment of the electron beam therewith. For this purpose, various adjustment coils 127-130 are provided. Thus, at the beginning of the exposure, the electron beam is turned on and the arrival of the electron beam at the stage 126 is detected while controlling the adjustment coils 127-130. During this procedure, the mask 110 may be removed from the optical path O for free passage of the electron beam. Alternatively, a large aperture formed in the mask 110 for passing the electron beam freely may be used.
In FIG. 1, it should be noted that the illustrated state represents the operational state of the electron beam exposure system wherein the electron beam is focused at the surface of the substrate 123. In this state, the focusing point f1 of the optical system formed of the lenses 108 and 116 is located above the blanking aperture plate 117 for achieving the desired demagnification.
In order to control the exposure operation, the electron beam exposure system of FIG. 1 includes the control system 200, wherein the control system 200 includes memory devices such as a magnetic tape device 201 and magnetic disk devices 202, 203 that are provided to store various data of the device pattern of the semiconductor device to be written. In the illustrated example, the magnetic tape device 201 is used for storing various design parameters, the magnetic disk device 202 is used for storing the exposure pattern data, and the magnetic disk device 203 is used for storing the pattern of the apertures on the block mask 110.
The data stored in the memory devices is read out by a CPU 204 and transferred to an interface device 205 after data decompression. There, the data for specifying the pattern on the block mask 110 is extracted and stored in a data memory 206. The data stored in the data memory 206 is then transferred to a first control unit 207 that produces the foregoing control signals SM1-4 and supplies the same to the deflectors 111-114. Further, the control unit 207 produces and supplies a control signal to a mask moving mechanism 209 that moves the block mask 110 in the direction transverse to the optical path O. In response to the deflection of the optical beam by the deflectors 111-114 and further in response to the lateral movement of the block mask 110, one can address the desired aperture on the mask 110 by the electron beam.
The first control unit 207 further supplies a control signal to a blanking control unit 210 that in turn produces a blanking signal for shutting off the electron beam. This blanking signal is then converted to an analog signal SB in a D/A converter 211 and the analog signal SB is supplied to a deflector 115 that causes a deflection of the electron beam away from the optical axis O. In response to this, the electron beam misses the blanking aperture 117a and disappears from the surface of the substrate 123. Further, the control unit 207 produces a pattern correction data H.sub.ADJ and supplies the same to a D/A converter 208. The D/A converter 208 in turn produces a control signal S.sub.ADJ and supplies the same to a deflector 106 that is provided between the electron lens 107a and the electron lens 107b. Thereby, one can modify the shape of the electron beam that has passed through the addressed aperture in the mask 110. This function is used when the desired shape of the electron beam is different from the shape given by the apertures on the block mask 110.
The interface device 205 further extracts and supplies the data for controlling the movement of the electron beam on the surface of the substrate 123 to a second control unit 212. In response thereto, the control unit 212 produces a control signal for controlling the deflection of the electron beam on the surface of the substrate 123 and supplies the same to a wafer deflection control unit 215 that in turn produces and supplies deflection control signals to D/A converters 216 and 217. The D/A converters 216 and 217 in turn produce drive signals SW1 and SW2 for driving the deflectors respectively and supply the sam to the deflectors 124 and 125 for causing the deflection of the electron beam. Thereby, the position of the stage 126 is detected by a laser interferometer 214 and the wafer deflection control unit 215 modifies the output deflection control signals and hence the drive signals SW1 and SW2 according to the result of measurement of the stage position by the laser interferometer. Further, the second control unit 212 produces a control signal that causes a lateral movement of the stage 126.
In such a conventional block exposure system, there arises often a need for checking whether the pattern of the apertures on the block mask 110 is defect-free or not. As the size of each pattern on the block mask 110 is in the order of several hundred microns, and as there are tens and hundreds of apertures provided on the mask 110, there is a substantial risk that one or more of these apertures may be defective. Such a defect may be caused at the time of the fabrication process of the block mask 110 or during the use of the block mask 110, in which the thin membrane of the block mask 110 is deformed by the energy of the electron beam. Thus, it is necessary to conduct a checking of the block mask 110 prior to the exposure process. Further, such a checking routine may be made with a regular interval during the exposure process.
Conventionally, such a checking of the aperture pattern of the block mask 110 has been done by a microscopic observation. There, the block mask 110 is dismounted from the optical system 100 of the electron exposure system and brought under the optical microscope. However, such a checking routine is time-consuming and increases the risk of the block mask 110 being damaged particularly during the dismounting and mounting process. Further, such a conventional checking process cannot detect any dirts that have been deposited on the mask 110 after the microscopic observation.